In order to perform read, program and erase operations in Flash memory, it is generally necessary to apply high voltages (higher than the typical control logic VCC supply) to the Flash cell. However, consumers are demanding higher speeds and lower power consumption. Higher speeds, of course, typically require faster transistors. One technique for designing and manufacturing faster transistors may be to decrease the distance between the transistor gate and the drain. However, this may create a high voltage overstress problem. As the distance between the gate and the drain diffusion decreases the maximum operating voltage of the transistor may decrease. If the distance is too small the silicon barrier between the gate and drain may have an excessive leakage current or may break down completely. Conversely, if the power supply voltage is lowered then the transistors may be made smaller and faster.
Also, if the operating voltage is decreased by some factor but the current drawn by the transistor does not increase by more than that factor, then the power consumption may decrease or at least not increase.
Therefore, making smaller transistors generally results in faster transistors and reduced power consumption. However, a Flash memory typically requires a high voltage to program, read and erase the Flash memory cell. Therefore, a level shifter circuit may be used to interface between the low voltage requirements of the control circuitry and the high voltage requirements of the Flash memory cell.
A level shifter typically takes a low voltage input, usually a logical voltage, and level shifts it to a high voltage. Two primary types of level shifters are currently in use. The traditional level shifter uses a cross-coupled P-devices formation. This type of level shifter is typically the fastest and consumes the least power. A ratioed logic level shifter is smaller but consumes more power than the traditional level shifter.
However, a problem may arise when using conventional level shifters between the low voltage, fast transistor logic circuitry and a high voltage Flash memory: the level shifters are in the control path for the Flash memory and, if made entirely of high voltage devices, may be slower. This adversely impacts the memory read, write and erase times, and therefore may adversely impact product performance.